1. Field of the Invention
This invention relates to a gradation control circuit in general, and more particularly to a gradation control circuit for a thermal head used for reproducing image signals including photographic information concerning a picture or the like.
2. Description of the Prior Art
Japanese Laid-Open Patent Publication No. 57-48868 (48868/1982) in the name of Masatoshi Katoh, entitled "thermal recording device", relates to a control circuit for a thermal head which can be utilized for gradation printing and is herein schematically illustrated in FIG. 1 as one typical conventional example useful for explaining the background of the present invention.
In FIG. 1, a binary recording circuit generally designated by the reference numeral 38 comprises a plurality of exothermic bodies 20a-20c each of which corresponds to one dot or pixel of an image to be printed or reproduced, a plurality of switching elements 23a-23c used for driving or energizing the exothermic bodies 20a-20c, a shift register 33 having a plurality of register stages corresponding to the number of dots and acting as a series-parallel converter, and a latch circuit 35 used for holding output signals from the shift register 33. A gradation recording circuit 34 comprises a reference signal generation circuit 31 for generating a set of reference signals consisting of a signal (0) which is continuously maintained at low level and signals (1)-(7) which are maintained at high level and differ in length respectively, and a data selector 32 having a group of input terminals 21 which receives a gradation code signal of 3 bits from a gradation code signal generator (not shown). Each gradation code signal corresponds to a gradation level for each of the exothermic bodies 20a-20c and is generated repeatedly at a cycle corresponding to a unit-heating time.
In this circuit arrangement, the number of reference signals from the reference signal generator 31 corresponds to the number of steps to be controlled (herein after referred to as gradation number or step), and the density or the degree of gradation of an image to be printed is determined by the number of times that one exothermic body corresponding to one dot or pixel is heated for its unit-heating time.
In the gradation recording circuit 34, when the gradation code signal is inputted into the data selector 32 via the input terminals 21, the data selector 32 selects the corresponding reference signal based on the gradation code signal of 3 bits and generates a binary signal which goes high for only a time interval of the gradation code signal provided.
In response to a shift clock signal provided at the terminal 36, the shift register 33 receives the binary signal selected at the data selector 32. Next, in response to data transfer pulses provided at the terminal 37, the latch circuit 35 fetches the output signals in the respective stages of the shift register 33 and latches them in the corresponding locations. The output signals of the latch circuit 35 are applied to the base electrodes of the switching transistors 23a-23c. Since the emitters and the collector electrodes of transistors 23a-23c are connected respectively to ground and the exothermic bodies 20a-20c, which are connected to a terminal 111 to which a recording voltage E is applied, the corresponding exothermic bodies will be energized according to the control signals from the latch circuits 35.
The complete recording of one dot or pixel is accomplished by dividing the heating pulses into a predetermined number (in this case, 9) of unit pulse, controlling the number of unit-heating pulses with reference to the gradation code signal provided, and thereby varying the heating time to the exothermic body.
However, in the conventional example mentioned above, when one picture element is to be printed, it is necessary to transmit gradation data to the shift register 33 for the same number of times as the gradation number. For this reason, assuming that the number of dots is 640 and the gradation number is 64, the printing speed per line can be expressed as (640.times.64)/(shift clock frequency), so that a time interval of 10.24 msec will become necessary provided that the shift clock frequency is 4 MHz. In addition, if the number of pixels is 640 and the gradation steps are 256, it amounts to 40.96 msec. Therefore, the printing speed obtained by the conventional circuit as illustrated in FIG. 1 cannot be applied to a high speed printer in which the printing time for one line must be limited to less than several seconds, unless the gradation steps are reduced.